The packaging industry makes and uses many sheet structures in the making of heat sealed packages. Such packages find use in a multiplicity of applications. Of particular interest to the invention are those applications where the finished package may be subjected to relatively abusive shocks, as by being dropped, bumped, repeatedly flexed, or the like. In cases where the package is holding fluid contents, the shock may be transferred to all parts of the package by the contained fluid. The transferred shock is effective to cause failure of the package at its weakest point when the severity of the shock is sufficiently great.
Failures in heat sealed packages tend to occur at or close to a heat seal. Some failures occur by partial peeling of the facing sealant layers from each other in the seal area. Where the shock is severe enough, the package may have a catastrophic failure wherein the package may be penetrated to the outside. Such penetration resulting from shock abuse typically occurs at or near the seals. Such failure of the package, whether by partial peeling or by catastrophic failure, compromises the containment and protective functions of the package, and is thus not acceptable.
It is desirable to find material compositions which may be used in packaging sheet structures which compositions enhance the capability of the sheet structure, and particularly the sealant layer to withstand increased amounts of shock-type abuse, while providing the capability in the outer surface of the package to tolerate direct contact with other packages, at conditions normally anticipated to be encountered over the life of the package, without the packages sticking to each other. These specific capabilities are functionally effective only to the extent they are combined into a package which otherwise exhibits capabilities compatible with the instant requirements.
In certain packaging applications, the packages are subjected to heat treatment, such as with steam or hot water, at elevated temperatures of about 121.degree. C. as part of the packaging process. Such heat treatments, commonly known as retort processing, are common in certain packaging of medical supplies and shelf stable foods.
Polypropylene polymers, which are otherwise excellent for use with high temperature applications, tend to become more brittle after being subjected to such processes, especially those processes in which it is subjected to steam or hot water. Increasing brittleness leads to less ability to resist shock-type abuse. While the use of such materials is desirable for their ability to withstand heat treatment without failure of the package during the heat treatment, the resulting brittleness is an undesirable feature, as it affects the ability of the package to withstand the shocks of, for example, shipping and handling.
There is also concern that the package may be subjected to repeated flexing during its life cycle. While polypropylene is desirably tolerant of the high temperatures used for processing medical supplies and shelf stable foods, after having been subjected to such high temperature retort processing, it tends to be less resistant to cracking under the repeated flexure. In some cases, the cracks are seen as a direct result of repeated flexing. In other cases the cracking may be exhibited as a result of shock-type physical abuse. Exemplary of such abuse are jiggling, repeated bumping, and high impact dropping.
The susceptibility of the overall package to failure as a result of cracking is a property which is a composite of, at least, the tendency of the individual layers to crack, the interfacial bond strengths, the elasticity of the adjacent layers and their capability to absorb and dissipate physical shock, the thickness of individual layers, and, to a degree, any overwhelming tendency of any one layer to crack. This tendency toward cracking is sometimes designated as brittleness.
The composite evaluation of the ability of retort processed packages to tolerate abuse is usually observed as the fraction of the filled and sealed packages which fail after being subjected to some physical stress or shock.
While polypropylene has been accepted as the material of choice for use in sealant layers of retortable pouches in many instances, it has been selected more or less by necessity, as few other materials in the same price range can provide similar high temperature processability. Likewise certain polypropylenes are among the few materials which are approved for contact with food and medical solutions. Thus, polypropylene provides a combination of desirable properties of tolerance of high temperature and approval for food contact, at a modes cost.
The shelf life of medical solutions and shelf stable foods is influenced, in part, by the stability of the moisture content of the contained product. Thus the ability of the package to retard, or prevent, transmission of moisture vapor through the packaging material may have a direct influence on the length of the shelf life of the contained product.
Polypropylene provides a good barrier to transmission of moisture vapor.
It is an object of this invention to provide a multiple layer polymeric film suitable for use in forming a package for shelf stable foods and medical solutions. both of which may be subjected to retort processing at elevated temperatures of, for example, about 121.degree. C.
It is a further objective to provide such packages which have improved capability to tolerate flexing and other stress crack inducing activities.
It is a further object of this invention to provide multiple layer films incorporating therein novel compositions; and packages made from those films.
It is yet another object to provide packages made from the novel films herein, which particularly have been retort processed at temperatures of about 121.degree. C., and for about 30 minutes of processing at that temperature.